There's an old story, possibly apocryphal, about one of the minor actors on the long-running US comedy MASH.
This series was set in an army combat hospital during the Korean War of 1950-52. The show, hugely popular in the 1970s and early '80s, starred - among other talented actors - Alan Alda as a capable but undisciplined medical doctor.
Once, according to legend, someone asked the actor who played the hospital chaplain, a minor role, what the show was about.
"It's about this guy," he said, "who's a priest in an army hospital in Korea during the war. Oh, and there are some doctors in it, too."
Close, I guess. But he was looking at the wrong part of the frame. The person in the centre of the frame is the one the thing is about. The person in the fuzzy background is not who the thing is about. Supporting cast members often forget that.
But even co-stars can forget who is really at the top of the cast list. Buddy pictures - movies and TV shows about a friendship, such as Butch Cassidy and the Sundance Kid, or the long-running TV comedy Laverne & Shirley - are what we in Hollywood call "two-handers".
There are other characters, naturally, but the storylines are about the team, the friendship, and the adventures they face together. Both characters are essential.
And so the names of the two main actors appear in the credits together, side by side, at the same time. It's what we call a "shared card" and it's not enough that the card be shared. In a two-hander, everything must be equal, even the font size of the actors' names in the credits.
All of these things are laboriously negotiated by agents, managers and lawyers, none of whom want to report back to their client that the project, once billed as a two-hander, has become, in terms of contractual things like billing and episode fee and font size, more of a "star/co-star" situation.
Which brings us to the most complicated buddy picture of all, the epic two-hander called the US presidential campaign.
By now, President Barack Obama has made his peace with his zany and unpredictable cast-mate, Vice President Joe Biden. Not a day goes by, though, that someone in the press isn't listing Mr Biden's various gaffes - mixing up the names of states, lapsing into racially-charged rhetoric - and wondering aloud if, perhaps, it wouldn't be prudent for Mr Obama to indulge in the time-tested tradition of recasting one of the leads.
Secretary of State Hillary Clinton springs to mind. It's always easier to think about recasting when you know there's another star available to step into the role.
But that's also part of the problem: a US presidential campaign only seems like a two-hander. It's really a star/co-star kind of thing. And no star wants to cast someone a little more popular and charismatic in the second lead, which is why most (successful) vice presidents have tended to be colourless order-takers.
The Republican candidate for president, Mitt Romney, has pretty much locked down the grey and colourless vote, which made his choice of a cast-mate even harder. It was unlikely he'd find someone more bland than himself.
He went in another direction, casting Congressman Paul Ryan as his co-lead. Ryan is a young, dynamic and often controversial - in other words, he's a star. Like him or hate him, Ryan is a consequential figure whose ideas get a lot of press attention. As the campaign heats up, it may be hard to remember who's supposed to get top billing.
That's the problem with politicians: they're amateurs when it comes to show business.
Once, not too long ago, a TV actor was being troublesome and recalcitrant. This actor was a supporting player in an ensemble comedy headlined by a famous comedian.
One day, after a particularly unpleasant outburst by the cast member, the star, whose name appeared in the title of the show - asked the difficult cast member to take a walk and talk things out.
They did, and the actor poured out a litany of petty complaints - not enough material, not enough episodes devoted to his character . The star whose name was in the title listened silently.
But when they returned to the soundstage, with its giant sign with the star's name, in front of an enormous door with the star's face, next to the star's reserved parking space, the star, still silent, simply pointed to the sign, the door, and the parking space. Then he stared at the actor and said quietly, "When that's your name, then you can open your mouth."
Actually, the legend is that the star inserted a common Anglo-Saxon vulgarism between the words "your" and "name", "can" and "open", and "your" and "mouth", which would have made the statement a lot more memorable.
Mitt and Barack would do well to prepare a similar speech. All co-stars eventually get out of hand.
Rob Long is a writer and producer based in Hollywood. On Twitter: @rbcl